Most marine outboard, inboard, and inboard-outboard propulsion motors utilize a raw water-cooling system. Raw lake or sea water is drawn into the motor by a water pump or the movement of the boat to provide an active cooling process for the motor. The water is circulated through fluid cooling jackets of the motor in order to cool the motor, and the water is returned to the lake to dissipate the heat generated by the internal combustion occurring within the motor.
At the propulsion end or lower unit, marine motors generally incorporate an oil-filled gearbox containing gears that provide rotation for the propeller to provide propulsion for the boat. The gearbox operates while submerged in lake water. The propulsion end or lower unit generally includes an intake to supply cool water for “actively” cooling the engine. The water enters the intake, passes up through the lower unit, and about the engine's cooling jackets in order to cool the engine.
These conventional marine motor cooling systems are unable to regulate or control how much heat is dissipated from the motor. Consequently, in many (if not most) situations, the motor is being operated at a temperature below the optimum operating temperature of the motor. The active cooling is especially detrimental for the performance and operation of the motor during warm-up, a time when cooling should be halted.
Additionally, water within the fluid cooling jackets of a marine motor has an undesirable destructive effect on the motor. Water causes rust, scaling, corrosion, metal degradation by electrolysis, and fracture by freezing. These problems are amplified when the motor is operated in salt water. Operators are also bothered with draining these water-cooling systems to prevent damage from ice if the motor is stored or transported in freezing climates. Generally, many motors, especially the inboard-outboard motors, require the operator to winterize their motor by draining all the water from the cooling system. Salt water systems have to be regularly flushed with fresh water.
A new problem related to marine water-cooling systems has recently came into focus. Recreational boats unintentionally transport and spread unwanted invasive species throughout our country's lakes and rivers. Zebra muscles or other invasive species may be drawn into the cooling system and then migrate to another body of water by traveling in the residual cooling system water in the boat motor.